1. Field of the Invention
The present invention relates generally to a communication device and method for a CDMA communication system, and in particular, to a data communication device and method for assigning a designated forward channel to send packet data to a mobile station.
2. Description of the Related Art
In general, CDMA (Code Division Multiple Access) communication services are classified into a circuit exchange service and a packet service. In the packet service, data is generated in a burst mode rather than continuously. Taking into consideration a radio resource capacity, a capability of a base station and a limited power consumption of a mobile station, the system need not continuously assign a dedicated traffic channel and a dedicated control channel to each user during the service. For this reason, a dedicated channel is temporarily assigned to the user when the burst traffic (or data) is generated, and is then released after transmission of the packet data, to thereby enable other users access to the limited channel resource.
FIGS. 1-4 illustrate various aspects of conventional packet data communication. The term “common channel” is used hereinbelow, with reference to FIGS. 1-4 to describe a channel which is used in common by several mobile stations both in reverse and forward links. For example, the common control channels include a forward paging channel, a forward common control channel, a reverse access channel and a reverse common control channel. In the third generation mobile communication radio transmission technology text, drafted by CDMA one group, forward common channel have forward pilot channel, synch channel, paging channel and forward common control channel. Reverse common channel have access channel and reverse common control channel.
FIGS. 1 and 2 are diagrams illustrating conventional procedures for assigning a dedicated channel. FIG. 1 illustrates a procedure for assigning the dedicated channel when a mobile station (MS) is in a suspended state, and FIG. 2 illustrates a procedure for assigning the dedicated channel when the mobile station in a dormant state. When the data service has not been exchanging user data for some period, dedicated channels are released for other users. When user data is generated and needs to transmit to the mobile station, the base station has to reassign dedicated channels with exchanging control data on common control channels.
Referring to FIG. 1, for a mobile station in a suspended state, when there is packet data to transmit to the mobile station, the base station sends a resource allocation message via a forward common control channel. Upon receipt of the resource allocation message, the mobile station negotiates with the base station about assignment of dedicated traffic channels, via forward and reverse common channels. When the dedicated traffic channels are assigned according to the channel negotiation, the base station and the mobile station exchange user data via the assigned forward and reverse traffic channels.
Referring to FIG. 2, for a mobile station in a dormant state, when there is packet data to transmit to the mobile station, the base station sends a paging message via a forward common channel, and the mobile station then sends a response message in answer to the paging message via a reverse common channel. Upon receipt of the response message, the base station sends a channel assignment message via a forward dedicated control channel. Thereafter, the base station and the mobile station initiate encryption and perform service negotiation and RLP (Radio Link Protocol) synchronization, via the forward and reverse dedicated control channels. Subsequently, the base station and the mobile station perform forward link scheduling via the forward and reverse dedicated control channels, and exchanges user data via forward and reverse dedicated traffic channels assigned according to the service negotiation.
With reference to the channel assignment procedure described above, when dedicated traffic channels are re-assigned between the base station and the mobile station, latency and signaling overhead occur due to re-negotiation performed prior to exchange of the packet data. Here, the overhead which occurs in the process of assigning the dedicated traffic channels includes synchronization overhead for the RLP and signaling overhead pertinent to the service negotiation for re-connection of the packet service.
In addition to the aforementioned procedures, it is also possible to transmit the packet data using the common channel instead of the dedicated control channel. A method for transmitting the packet data using the common channel is illustrated in FIG. 3.
Referring to FIG. 3, when there is packet data to transmit to the mobile station, the base station notifies the mobile station that there is packet data to transmit, via a forward common control channel. Upon receipt of the notice, the mobile station sends a response message via a reverse common control channel. Upon receipt of the response message, the base station assigns to the mobile station one of several time slots constituting a radio frame used for a common channel, and sends a corresponding channel assignment message via the forward common control channel. Upon receipt of the channel assignment message, the mobile station sends a response message via the reverse common control channel, and upon receipt of the response message, the base station repeatedly sends the packet data by loading it on a corresponding time slot of a common traffic channel until there is no more packet data to send.
In this case, the transmission time required for sending the packet data on a common channel, exclusive of the signaling overhead, can be defined as
 Total Transmission Time=(N/Nb)×(T+T0)  (1)
where:
                N denotes the total number of data bits to be transmitted,        Nb: the number of data bits transmitted during one time slot,        T0: a length of one time slot, and        T: is a time delay until next time slot.        
That is, transmitting the packet data using the common channel causes an increased time delay as defined by equation (1) in addition to the signaling overhead due to channel assignment.
FIG. 4 is a diagram illustrating a conventional procedure in which the mobile station receives a response (or acknowledge) signal from the base station via a paging channel (PCH).
Referring to FIG. 4, the mobile station sends a control message requesting a response signal via a reverse common control channel. Upon receipt of the control message, the base station awaits a time slot assigned to the corresponding mobile station and then sends the acknowledge signal at the above time slot via the paging channel (PCH), thereby resulting in an undesirable time delay.